Don't add CYA yet until you get your pool cleared up.

Did you test your pH when the chlorine level was at 17ppm? If so, then your pH reading is probably inaccurate. Testing it at high CL levels usually gives falsely high pH readings. Retest it when the chlorine is lower.

Did you do the overnight test? That is an important test to do and to report here. Chlorine loss during the day may be to sunlight, but chlorine loss overnight means you are fighting something in the water.

Add bleach this evening after the sun is off the pool -- enough to get the chlorine level back up to around 12-15. An hour after adding it, test the chlorine. Then, test it again within two hours of sunrise in the morning. Then, report back here. Be sure you also test and report the CC reading in the morning.

Do not use your SWCG right now. Wait until this is cleared up.